Final answer:
The in-store purchase offered a better deal for Sally because the cost per item was lower compared to the online purchase.
Step-by-step explanation:
To determine which place offered the better deal for Sally's purchase, we need to compare the cost per item for the online and in-store purchases.
For the online purchase, Sally bought 18 items for $42. Therefore, the cost per item is $42/18 = $2.33 per item.
For the in-store purchase, Sally bought 12 items for $20. Therefore, the cost per item is $20/12 = $1.67 per item.
Comparing the two costs per item, we can see that the in-store purchase offered the better deal because the cost per item was lower compared to the online purchase.